Sec. 6. Systems of Geodetic and Geographic Coordinates

The position of the horizontal projections of the points of the earth's surface on the level surface MN can be determined using coordinates taken in a selected frame of reference. Coordinates are quantities which locate the position of a point on a surface or in space with

Fig.4

respect to an adopted coordi­nate system. To locate points on the earth's or spheroid's surface it is

usual to employ a geodetic, an astronomical or a geographic coordinate system. These are unified coordinate systems for all points on the earth's systems. The two basic references for them are the plane of the initial meridian PA₀P₁ and the plane of the equator EA₀E' (Fig. 4). The meridian passing through Greenwich a suburb of London, is considered to be the initial meridian, that is the international zero of longitude. The position of any point A on the earth's ellipsoid in the geodetic co­ordinate system is determined by two numbers. These are the geodetic latitude, the angle φ between the normal AO to the surface of the earth's ellipsoid at the point and its equator's plane, and the geodetic longitude. The latter is the dihedral angle λ between the plane of the geodetic meridian PAP₁ that passes through the given point and the plane of the initial geodetic meridian PA₀P₁ (see Fig. 4). The geodetic meridian is a plane that passes through the normal to the surface of the earth's ellipsoid at the given point and is parallel to the ellipsoid's minor axis. If the angle φ is between a plumb line at the given point and the plane at right angles to the axis of the Earth's rotation, then it is called the astronomic latitude. The dihedral angle λ between the plane of the astronomic meridian through the given point and the initial astronomic meridian is the astronomic longitude of the point A. A generalization of the geodetic and astronomic coordinates when the deviation of the plumb lines need not be taken into account are expressed as geographic coordinates. Geodetic, astro­nomic and geographic coordinates are measured from 0 to 90° start­ing at zero at the equator and increasing toward the poles. Latitudes φ reckoned from the equator northward are termed north latitudes whilst those reckoned southward are south latitudes. Longitudes λ are measured from the initial (zero) meridian eastward and westward and go from 0 to 180°. They are termed, respectively, easterly and westerly longitudes. Geodetic and geographic latitudes and longitudes are the principal coordinates in locating points on the ground. Geodetic and astronomic coordinates can be established for any given point using astronomic and gravimetric observations. The height above the surface of the earth ellipsoid is the third coordinate of a point on the earth's physical surface.

Heights that are measured from a level surface concurrent with sea level (the datum level) are termed geodetic heights while those given with respect to an arbitrary level surface parallel to MN are called assumed heights. It is customary to regard mean sea level (MSL) as the starting point for measuring geodetic coordinates. In the USSR it is usual to measure geodetic heights from the zero of the Kronstadt tidal gauge. This latter is a plate of copper carrying a horizontal bar embedded in the abutment of the Obvodnyi bridge. The horizon­tal bar represents the zero of the gauge.